There are a variety of applications in which it is desirable to limit the flow of current to conserve power. For example, continuous direct current flow between two remote stations indicates the supervisory states of the two stations. Interruptions in the current flow are used to send information from one station to the other. To avoid custom design for every installation, terminal apparatus must be capable of operation with interconnecting transmission paths of different resistances. Equipment is designed so that for a maximum resistance path the current flow in the path will be the minimum required for reliable activation of the sensing elements utilized in the equipment. Installations having less than this maximum resistance path between stations will allow more current to flow than is necessary, accordingly, power is wasted. Interruptions in the direct current flow can be provided by metallic contact devices or by solid state devices. Factors such as size, cost, circuit isolation, and long life favor the use of solid state devices.